In a typical, well-known automatic transmission for an automobile, the rotation of an engine is input via a torque converter, speed-varied by a shift mechanism having a plurality of planetary gears, and then output to a drive shaft or a propeller shaft (axle side).
The shift mechanism in this type of automatic transmission executes a shift by transmitting the rotation of an input shaft to a specific gear or carrier of the planetary gear in accordance with a shift position, and transmitting the rotation of the specific gear or carrier to an output shaft appropriately. Further, a plurality of friction elements such as clutches and brakes are provided to constrain the rotation of the specific gear or carrier appropriately during the shift, and a predetermined shift is performed by switching a transmission path through engagement and disengagement combinations of the friction elements. Furthermore, hydraulic clutches and brakes, the engagement state of which is controlled in accordance with an oil pressure supply/discharge state, are normally used as the friction elements.
When a predetermined shift is performed in a conventional automatic transmission while the vehicle is traveling near a boundary region of a vehicle traveling condition, a selected gear position varies such that the shift is performed repeatedly. For example, when a 3-4 shift is performed from a third speed to a fourth speed, a 3-4 shift from the third speed to the fourth speed and a 4-3 shift from the fourth speed to the third speed are performed repeatedly, leading to a continuous shift in the form of 3-4-3-4- . . . .
When a shift is performed continuously in this fashion, the same friction element is engaged and disengaged repeatedly over a long time period, and therefore a thermal load applied to the friction element increases (the temperature rises). As a result, the friction element may suffer burnout. It should be noted that in this specification, the term “thermal load” is used to mean “temperature” or “generated heat”.
In response to this problem, JP3402220B discloses a technique employing a timer. More specifically, the timer is counted down during a continuous shift, and when the timer value reaches a predetermined value, subsequent shifts are prohibited under the assumption that the thermal load state (temperature) of the friction element has reached a burnout temperature. Further, when the continuous shift ends before a set value has been reached, the timer is counted up on a constant gradient under the assumption that heat radiation is underway.
Hence, when a continuous shift resumes immediately after the end of a continuous shift, the countdown begins from a smaller value than the initial value of the timer value, and thus control is executed taking into account an accumulated heat amount of the friction element.